1. Technical Field
The disclosed embodiments relate to transformers for high frequency applications.
2. Background Information
Radio frequency (RF) circuits such as transceivers found in cellular telephones often involve inductors and transformers. FIG. 1 (Prior Art) is a top-down view of a spiral-shaped inductor 21 disclosed in U.S. Pat. No. 6,180,445. Spiral-shaped inductor 21 is fabricated on a silicon substrate along with active devices in an active region of the silicon substrate. An inductor may also be partly disposed in a non-semiconductor substrate and connected by a microbump to circuitry in an IC, which can then be flip-chip bonded to a BGA package substrate.
U.S. Pat. No. 6,833,285 shows another example of a spiral-shaped inductor that is fabricated on a silicon substrate. The structure is part of a flip-chip silicon interposer. FIG. 2 (Prior Art) is a top-down view of a flip-chip transformer 16 that is disclosed in U.S. Published Patent Application US2004/0121606. Transformer 16 is fabricated on a semiconductor wafer 2. FIG. 3 (Prior Art) is a perspective view of an integrated transformer structure. The transformer involves three primary nodes P1, P2 and P3 and two secondary nodes S1 and S3. The primary nodes are connected to a mixer of a transmitter. One of the secondary nodes is connected to an input of a driver amplifier whereas the other of the secondary nodes is grounded. The mixer and driver amplifier and transformer are parts of a flip-chip packaged transceiver integrated circuit available from Qualcomm Incorporated. Prior art integrated transformer structures, for example such as the structures illustrated in FIGS. 2 and 3, generally have either an undesirably low quality factor Q, and/or an undesirably low coupling coefficient, and/or occupy an undesirably large amount of integrated circuit die area. An alternative, that has a high Q, an adequately high coupling coefficient, and that occupies a relatively small amount of integrated circuit die area is desired.